1. Field of the Invention
This invention generally relates to a method of fabricating a semiconductor device having basic portions made of silicon carbide (SiC). This invention specifically relates to a method of fabricating, for example, a SiC MOSFET.
2. Description of the Related Art
There is a known semiconductor device including a SiC substrate or a 4H—SiC substrate. In the case where an oxide film (an SiO2 film) formed on a SiC substrate, especially a 4H—SiC substrate, is used as a gate oxide film, an interface level density is extremely high so that a channel mobility decreases. It is thought that the high interface level density is caused by impurity such as carbon which remains in an SiO2/SiC interface between the gate oxide film and the SiC substrate. When the SiC substrate is thermally oxidated to form the gate oxide film, the oxidization causes carbon to remain in the SiO2/SiC interface as impurity. On the other hand, in the case where the gate oxide film is formed on the SiC substrate by deposition, the SiC substrate is exposed to the atmosphere after being treated with HF. In this case, carbon in the atmosphere adheres to a surface of the SiC substrate, and then an SiO2 film is deposited on the surface of the SiC substrate as the gate oxide film. Therefore, carbon remains in the SiO2/SiC interface as impurity. It is thought that such impurity causes the high interface level density.
Japanese patent application number 2001-17263 which is not prior art to this invention relates to a method of fabricating a SiC semiconductor device. The method in Japanese application 2001-17263 includes a step of executing thermal oxidation at a high temperature to reduce the amount of carbon remaining in an SiO2/SiC interface between a gate oxide film and a SiC substrate. The reduction in the amount of remaining carbon results in a decrease in an interface level density. According to the method in Japanese application 2001-17263, it is difficult to nullify the amount of remaining carbon.
Japanese patent application publication number P2000-133657A discloses a method of fabricating a SiC semiconductor device. According to the method in Japanese application P2000-133657A, a SiC substrate is thermally oxidated to form an oxide film, and an oxide/SiC interface occurs. The method is designed to decrease an interface level density at the oxide/SiC interface. Specifically, the method includes a first step of forming a silicon oxide film on one main surface of the SiC substrate, and a second step of treating the SiC substrate and the silicon oxide film with a hydrogen plasma atmosphere to reduce the interface level density at the oxide/SiC interface.
Japanese patent application publication number P2000-252461A discloses a method of fabricating a semiconductor device. According to the method in Japanese application P2000-252461A, a semiconductor substrate having an uppermost layer containing SiC is used, and a gate insulating film is formed on the semiconductor substrate. There is a gate-insulating-film/SiC interface. The gate insulating film includes at least one layer of an oxide or a nitride. The semiconductor substrate with the gate insulating film is annealed in an atmosphere at a temperature of 600° C. to 1600° C. The atmosphere contains hydrogen. As a result of the annealing in the hydrogen-based atmosphere, dangling bonds of carbon or silicon at the gate-insulating-film/SiC interface are terminated at hydrogen so that an interface level density is sufficiently reduced.
Japanese patent application publication number P2001-345320A which is not prior art to this invention relates to a method of fabricating a semiconductor device. According to the method in Japanese application P2001-345320A, a silicon oxide film (an SiO2 film) is formed on a semiconductor substrate of SiC by a chemical vapor growth method. There is an interface between SiO2/SiC. The method is designed to improve the quality of the silicon oxide film to provide a low interface level density at the SiO2/SiC interface. Specifically, to improve the quality of the silicon oxide film, the semiconductor substrate with the silicon oxide film is heat-treated in an inert gas at a temperature of 1100° C. to 1400° C. for 30 minutes or longer.